A vehicle may include a sensor for detecting objects around the vehicle. The sensor may be a radar system that transmits an electromagnetic wave and receives a reflected signal from an object. The relative speed and distance of the object may be detected based on the characteristics of the return signal. In order to operate properly, the radar must be able to transmit and receive the electromagnetic signal. Under some situations, the transmitted and received signals may be attenuated. Debris or obstructions in front of the radar unit may cause attenuation of the radar signals. For example, mud, ice, or snow in front of the radar unit may obstruct the radar function. Under these conditions, the return waves may be so degraded that a low radar return signal is measured. When a low radar return signal is detected, a radar diagnostic system may diagnose a radar fault condition and disable operation of radar related functions.